This invention relates to fluoropolymer processing additives that employ a vinylether monomer; to a melt processable thermoplastic polymer composition that utilizes this polymer processing additive; and to a method of improving the melt processability of a melt processable thermoplastic polymer.
For any melt processable thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. See, for example, R. F. Westover, Melt Extruision, Encyclopedia of Polymer Science and Technology, Vol. 8, pp 573-81 (John Wiley and Sons 1968). The desire for a smooth extrudate surface competes, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (i.e. at high shear rates).
Some of the various types of extrudate roughness and distortion observed in melt processable polymers, especially high and low density polyethylenes, are described by A. Rudin, et al., Fluorocarbon Elastomer Aids Polyolefin Extrusion, Plastics Engineering, March 1986, at 63-66. The authors state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins such as linear low-density polyethylene (LLDPE), high-density polyethylene (BDPE), and polypropylene suffer melt defects. At low shear rates, defects may take the form of xe2x80x9csharkskinxe2x80x9d, a loss of surface gloss, that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction. At higher rates, the extrudate can undergo xe2x80x9ccontinuous melt fracturexe2x80x9d becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from xe2x80x9ccyclic melt fracturexe2x80x9d, in which the extrudate surface varies from smooth to rough. The authors state further that lowering the shear stress by adjusting the processing conditions or changing the die configuration can avoid these defects to a limited extent, but not without creating an entirely new set of problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation.
There are other problems often encountered during the extrusion of thermoplastic polymers. They include a build up of the polymer at the orifice of the die (known as die build up or die drool), increase in back pressure during extrusion runs, and excessive degradation or low melt strength of the polymer due to high extrusion temperatures. These problems slow the extrusion process either because the process must be stopped to clean the equipment or because the process must be run at a lower speed.
Certain fluorocarbon processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. U.S. Pat. No. 3,125,547 to Blatz, for example, first described the use of fluorocarbon polymer process aids with melt-extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1:2 and wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers.
U.S. Pat. No. 4,904,735 (Chapman, Jr. et al.) describes a fluorinated processing aid for use with a difficultly melt-processable polymer comprising (1) a fluorocarbon copolymer which at the melt-processing temperature of the difficulty melt-processable polymer is either in a melted form if crystalline, or is above its glass transition temperature if amorphous, and (2) at least one tetrafluoroethylene homopolymer or copolymer of tetrafluoroethylene and at least one monomer copolymerizable therewith wherein the mole ratio is at least 1:1, and which is solid at the melt-processable temperature of the difficultly melt-processable polymer.
U.S. Pat. No. 5,397,897 to Morgan at al. describes the use of copolymers of tetrafluoroethylene and hexafluoropropylene having high hexafluoropropylene content as processing aids in polyolefins.
U.S. Pat. Nos. 5,064,594 to Priester et al., and 5,132,368 to Chapman, Jr. et al. describe the use of certain fluoropolymer process aids containing functional polymer chain end groups including xe2x80x94COF, xe2x80x94SO2F, SO3M, xe2x80x94OSO3M, xe2x80x94COOR and xe2x80x94COOM, wherein R is a C1-3 alkyl group and M is hydrogen, a metal cation, or a quaternary ammonium cation for use with a difficultly melt-processable polymer. The fluoropolymer is selected from the group consisting of (i) irradiated polytetrafluoroethylene, (ii) a partially crystalline copolymer of tetrafluoroethylene and a perfluoro(alkyl vinyl ether) or a perfluoroolefin containing 3-8 carbon atoms, (iii) an elastomeric copolymer of tetrafluoroethylene and a perfluoro(alkyl vinyl ether), (iv) a copolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene and (v) a copolymer of one or more fluoroolefins and 0.5-40 mole % of a functional-group-containing monomer 
wherein Z is xe2x80x94F or xe2x80x94CF3, x is 0 or an integer of 1-4, y is 0 or 1, z is an integer of 1-12, and Wxe2x80x2 is selected from the functional groups xe2x80x94SO2F, xe2x80x94SO2Cl, xe2x80x94SO3H, xe2x80x94COOR or xe2x80x94COOM, wherein R is C1-3 alkyl and M is hydrogen, a metal cation, preferably an alkali metal cation, or a quaternary ammonium cation, said fluoropolymer containing at least 100 functional groups W per million carbon atoms. Such functional groups are either thermally unstable or are chemically reactive to basic and/or acidic functionalities present in the extrudable resin or in adjuvants incorporated into the extrudable composition.
U.S. Pat. No. 5,464,904 to Chapman et al. discloses the use of unimodal semicrystalline fluoroplastics such as copolymers of tetrafluoroethylene and hexafluoropropylene and terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride with a polyolefin. The only enhancement of melt-processability described in this patent is shown in Example 25 where a concentration of 1000 ppm of the fluoropolymer in linear low density polyethylene is said to reduce the extrusion pressure of the extrudable composition. There is no showing of a reduction in melt defects.
U.S. Pat. Nos. 5,015,693 and 4,855,013 to Duchesne and Johnson disclose the use of a combination of a poly(oxyalkylene) polymer and a fluorocarbon polymer as a processing additive for thermoplastic hydrocarbon polymers. The poly(oxyalkylene) polymer and the fluorocarbon polymer are used at such relative concentrations and proportions as to reduce the occurrence of melt defects during extrusion. Generally the concentration of the fluoropolymer is present at a level of from 0.005 to 0.2 weight percent of the final extrudate and the poly(oxyalkylene) polymer is present at a level of from 0.01 to 0.8 weight percent of the final extrudate. Preferably, the weight of the fluorocarbon polymer in the extrudate and the weight of the poly(oxyalkylene) polymer in the extrudate are in a ratio of 1:1 to 1:10.
U.S. Pat. No. 5,710,217 to Blong at al. Discloses an extrudable thermoplastic hydrocarbon composition that comprises an admixture of a melt processable hydrocarbon polymer as the major component and an effective amount of a chemically-resistant fluoropolymer process aid. The fluoropolymer contains at least 50% by weight of fluorine and comprises one or more fluoropolymers that are essentially completely ethylenically unsaturated.
While prior processing additives have been useful, there is still a need to provide an improved fluoropolymer processing additive. It is desirable to provide a processing additive that gives improved melt processability as is discussed below. It is also desirable to provide a processing additive that is chemically stable towards aggressive host polymers (e.g., polar non-hydrocarbon host polymers such as nylon, etc.) and/or aggressive adjuvants employed in the host polymer (e.g., antioxidants such as HALS, etc.). Furthermore, it is desirable to provide a processing additive that is essentially free of groups that can degrade or undergo chemical interaction (e.g., hydrolysis, decarboxylation, etc.) during melt processing.
It has been discovered that a processing additive based upon a fluoropolymer that has been modified to include units derived from a perfluorovinyl ether as is described below is surprisingly effective in improving the melt processability of thermoplastic polymers.
The improvement in melt processability achieved by the present invention manifests itself in one or more ways. For example, the improvement may reduce, or eliminate, melt defects in a melt processed host polymer. It may postpone the occurrence of melt defects to a higher extrusion rate (i.e., shear rate) than is normally achieved without the use of the processing additive composition of the invention. It may reduce the occurrence of die build up experienced during the extrusion of the host polymer, especially non-hydrocarbon host polymers. This is advantageous because it reduces the amount of back pressure during extrusion of non-hydrocarbon polymers, and permits the use of lower extrusion temperatures to achieve an equivalent throughput.
Other advantages of the invention include the ability to tailor the melting point of the processing additive by varying the level of the perfluorovinyl ether employed in it. In a preferred embodiment of the invention it is possible to provide a processing additive having a melting point in the range of 60-200xc2x0 C.
Briefly, in one aspect, the present invention provides a polymer processing additive composition based on a fluoropolymer that contains interpolymerized units derived from a monomer composition of
(i) a perfluorovinyl ether of the formula
CF2xe2x95x90CFxe2x80x94(OCF2CF(CF3))AORfxe2x80x83xe2x80x83(I)
xe2x80x83where A has a value of from 0 to 3 (preferably from 1 to 3) and Rf is a fluoroaliphatic group, preferably a fluoroalkyl or a fluoroalkoxyalkyl group, of 1 to 8, preferably 1 to 3, carbon atoms.
(ii) a fluorinated olefinic monomer having the formula
RCFxe2x95x90CR2xe2x80x83xe2x80x83(II)
xe2x80x83where each R may be the same or different from each other and is selected from H, F, Cl, alkyl of from 1 to 8 carbon atoms or a perfluoro alkyl of from 1 to 8 carbon atoms;
(iii) a non-fluorinated olefinic monomer having the formula
CH2xe2x95x90CRxe2x80x22xe2x80x83xe2x80x83(III)
xe2x80x83wherein Rxe2x80x2 is H, Cl or an aliphatic group having from 1 to 8 carbon atoms.
The monomer composition is made up of from 0.1 to 10 (preferably from 1 to 8) percent by weight of the monomer of Formula I, from 60 to 99.9 (preferably from 70 to 95) percent by weight of at least one monomer of Formula II, and from 0 to 30 (preferably from 10 to 20) percent by weight of the monomer of Formula III. When the monomer composition contains no monomer of Formula III, it comprises from 0.1 to 10 percent by weight of the monomer of Formula I and from 90 to 99.9 percent by weight of two different monomers of Formula II.
The processing additive composition of the invention can be used per se. Alternatively, it can be combined with fluoropolymers that have not been modified with a perfluorovinyl ether. The performance of the non-modified fluoropolymer can be improved by blending a modified fluoropolymer with it so that the overall blend of the two fluoropolymers comprises 0.1 to 10 percent by weight of the monomer of Formula I.
In another aspect, the present invention provides a novel melt processable polymer composition that comprises a major amount (i.e., at least 50% by weight) of a melt processable thermoplastic host polymer and a minor, but effective, amount of the processing additive composition based on the perfluorovinyl ether-modified fluoropolymer.
In yet another aspect, the present invention provides a method for improving the melt processability of the host polymer. In this method the host polymer is combined with an effective amount of the processing additive composition. The resulting melt procesable polymer composition is mixed until there is preferably a uniform distribution of the processing additive composition in the host polymer. The polymer composition is then melt processed.
As used herein, an effective amount of the processing additive composition is an amount that improves the melt processability of the host polymer during extrusion over the melt processability of a host polymer that does not employ the fluoropolymer-based processing additive composition of the invention.